Signal amplifier



April 11, 1939. I E, C FREELAND 2,154,327

I SIGNAL AMPLIFIER Filed July 3, 1936 2 SheetsSheet l April 11, 1939. E, Q FREELAND 2,154,327

S IGNAL AMPLIFIER Filed July 3, 1936 2 Sheets-Sheet 2 Emu/sway 460m; 470M. 48mm Patented Apr. 11, 1939 UNITED STATES- PATENT OFFICE SIGNAL AMPLIFIER Application July 3,

Claims.

This invention relates to electrical signal amplifiers, and more particularly to high gain amplifiers, the principal object of the invention being "to provide a novel device of this class by which materially increased gain and other desirable results may be obtained. The invention is directed particularly to high-gain amplifiers in which either or both of the input and output circuits comprise resonant circuits adapted totransmit wave signals of certain frequencies and to attenuate signals of other frequencies. The invention may be employed, for example, in the intermediate frequency amplifier of a superheterodyne radio receiver, and for convenience it will be described with reference to such use, but it will be understood that the invention is not limited thereto and is capable of use in any instance where a high-gain amplifier may be desired.

The invention is also particularly adapted for use with vacuum tube amplifiers of the pentode type, which are characterized by having at least two electrodes positioned between the control electrode and the output electrode, one of the said intermediate electrodes being so biased with respect to the cathode that it draws no space current.

Another object of the invention is to provide a'high-gain amplifier stage including a vacuum tube having a high amplification factor and tuned inputand output circuits, which stage is: characterized by stability of operation and unusually high gain.

Still another object of the invention is to provide a tuned input and output circuit arrangement for an amplifier of the pentode type, by which an amplifier stage having band-pass filter characteristics may be obtained, and in which the attenuation of signals of undesired frequency is symmetrical with respect to a mean frequency.

A further object of the invention is to provide an input circuit for a pentode amplifier so constructed and arranged that the dynamic input admittance of the amplifier may be made substantially zero regardless of the nature of itsout- 5 put circuit; thereby eliminating certain defects or objectionable characteristics of the amplifier.

Still another object of the invention is to provide a simple and economical circuit for a. highgain tunedamplifier by which the amplifier may be stabilized.

As above indicated, a principal purpose of the present invention is to obviate certain objectionable characteristics of an amplifier caused by its dynamic input impedance. As is well known in the art, the input admittance of a vacuum tube 1936, Serial No. 88,853

amplifier having some sort of a load circuit, may be divided into two parts. One of these parts, which may be designated the static admittance, is determined by the physical dimensions of the tube elements and is the admittance which would be measured when the tube is deenergized. The second part, or dynamic admittance, is that part of the admittance which is determined by the flow of energy between the control grid and other elements of the tube and will, in general, depend upon the nature of the output circuit, the gain or amplification of the tube, and the inter-electrode capacities between the control grid and several other elements of the tube. In the usual operation ofa pentode amplifier, in which the screen grid and the suppressor grid are effectively connected to the cathode as far as alternating currents are concerned, and in which the anode is connected to a load circuit, the dynamic admittance will have the nature of a capacitance in shunt with a resistance. Where the ouput circuit has an inductive component, the conductance or real part of the dynamic admittance, i. e. that part of the admittance due to the shunt resistance will be negative and will correspond to a negative shunt resistance; while for a capacitive load, the conductance or real part of the dynamic admittance will be positive. The resistive componentof the load will, in general, reflect back and appear as a positive susceptance or as an eifective shunt capacitive element. If the input circuit for the tube is a parallel tuned circuit, it will be apparent that the dynamic admittance of the tube, which is eiiectively in shunt with the tuned input circuit and adds to the admittance thereof, will appreciably modify the characteristics of the whole circuit. For example, if the equivalent conductance of the tuned input circuit is less in absolute magnitude than the negative conductance of the dynamic admittance when the ouput circuit is inductive, the tube will oscillate of its own accord. Under less extreme conditions, the filter characteristics of the input circuit will be appreciably distorted. The present invention provides a simple and economical method by which the dynamic admittance of an amplifier may be reduced substantially to zero, thereby eliminating the above defects or objectionable characteristics.

The invention may be more fully understood by reference to the accompanying drawings and the following detailed description.

In the drawings:

Fig. 1 is a diagrammatic illustration of a tuned high-gain amplifier stage employing a preferred embodiment of the invention;

Fig. 2 represents the equivalent circuit of Fig. 1 rearranged in the form of a bridge circuit, by which a clear understanding of the operation of the circuit may be had, and in which the principal portion and elements of the circuit are shown by the heavy lines, while the less important portion and elements of the circuit are shown by lighter lines; and

Fig. 3 illustrates certain dynamic characteristics of the amplifier stage for several degrees of unbalance. I

Referring now to Fig. 1, the amplifier stage may comprise a tuned input circuit including a transformer T1, at tuned output circuit including a transformer T2, and a vacuum tube amplifier V of the pentode type. The transformer T1 may include a primary winding L1 shunted by a condenser C1 to form a tuned input circuit for the stage. The secondary winding L2 of the transformer may also be shunted by a condenser C2 to form another tuned circuit, which constitutes the input circuit for the tube V and is connected to the control grid, and through a blockingv condencer Cb to the cathode of the tube. In accordance with the invention, a tertiary winding L3 is connected to ground and to the suppressor grid of tube V for purposes which will. be described in detail hereinafter. L1 and L3 are inductively coupled to L2 but L1 need not be directly coupled to L3. The anode of the tube V may be connected to a source of electrical energy B through the primary winding L4 of the transformer T2, which winding may be shunted by a condenser C4 to form a tuned output circuit for the tube. The output signal of the stage may be obtained from the secondary winding L5 of the transformer T2, which secondary winding may be shunted by a condenser C5 to form a tuned output circuit. A biasing voltage may be applied to the control grid of the tube V by means of the isolating resistance R and the blocking condenser Cb, which condenser will have a negligible impedance for signals of the frequency which it is desired to transmit. The screen grid of the tube may be supplied with energy by means of the source B, which may also supply the anode, as above noted.

For convenience, it may be considered that the amplifier stage constitutes part of an intermediate frequency amplifier of a superheterodyne radio receiver and that it is designed to transmit wave signals in a certain fixed frequency band and to attenuate wave signals of other frequencies. In these circumstances, the primary land. secondary windings of transformers T1 and T2 would all be tuned to substantially the same frequency, and the coupling between the primary and secondary windings of each transformer might approximate critical coupling. It will be understood, however, that it is not necessary that the input and output circuits be of this nature. For example, the invention is well adapted for use in amplifiers of the radio frequency type, in which case the input transformer might comprise an untuned primary winding and a broadly resonant secondary winding adapted to transmit signals over a wide frequency band, and the output circuit might comprise any conventional load circuit.

To facilitate the following description, the electrodes of the vacuum tube V other than the cathode K, may be designated g1, g2, g3 and 94 in order of their spacing from the cathode foldesignate the inter-electrode capacitance between the elements g1 and 74. The capacitance between any of the said electrodes and the oathode will be designated by the subscript zero and 5 the number of the electrode. Thus, the cap-acitance between the control grid and cathode may be C01.

Considering for the moment the conventional operation of the device which would take place in the absence of the tertiary winding L3 and if as were connected to ground, it will be observed that the signal between g4 and the oath ode K or ground which may be designated Ep, will be dependent upon the signal between g1 and ground, which may be designated Egl, and in general the former signal will be of considerably greater magnitude than the latter. If the load circuit for or were purely resistive, the signal E would be exactly opposite in phase, that is 180 out of phase, with respect to the signal Egl and of considerably greater magnitude. For example, the signal Ep might be times the signal Egl. Due to the fact that there will inevitably be some slight capacity C14 between g1 and g4, signal energy will be transferred between g1 and g4. In the particular instance, this effect might be represented by introducing an additional equivalent capacity between or and ground having a value proportional to C14 multiplied by the voltage ratio between E1: and Egl. Thus, in the particular instance above noted, this effective capacity would be 100 times as great as Cn.

If, now, the load circuit contains an inductive component, the voltage Ep will be displaced from Eg by an amount greater than This voltage may be represented by two components one 180 and the other 270 out of phase with respect to Egl. That part of the voltage Ep which is 270 out of phase with respect to Egl will effectively introduce a negative conductance between 91 and ground, since it will cause energy to be transferred from the output circuit to the input circuit. If on the other hand, the output circuit contained a capacitive component, then the voltage Ep would be out of phase with respect to Egl by less than 180, and that part which was 90 out of phase would reflect back as an equivalent positive conductance between $71 and ground, since under such conductions, energy would be transferred from the input circuit to the output circuit. In both cases, that component of the voltage Ep which is 180 out of phase with respect to Egl will reflect back as an augmented capacitance or positive susceptance.

By means of the present invention, the input circuit for the amplifier may be so arranged that the control voltage Egl is independent of the voltage Ep, and hence no energy will be transferred from the output circuit to the input circuit or vice versa, except through the thermionic operation of the vacuum tube, and the input admittance of the tube will be independent of the nature of the output circuit. The manner in which the invention, as embodied in Fig. 1, accomplishes these results may be more clearly understood by reference to Fig. 2, wherein the circuit of Fig. 1 has been replaced by an equiv- 2,154,327 alent bridge circuit. The four corners of the bridge, which represent points of different potential in thevacuum tube V, considered in a clockwise direction are, g1, g4, g3 and K-gz. The last point corresponds to the potential of the cathode and that of the screen grid g2. While these two elements are at different D. C. potentials, with respect to alternating currents of the frequency which the amplifier is adapted to transmit, the two are at the same potential. The bridge arm between the points K-g2 and 91 includes the input circuit and of this it is necessary to show only the secondary L2 which, of course, is shunted by the condenser 02 and the tube'capacitance C01.

' will be shunted by the less important tube capacity series with an equivalent resistance Rpin magnitude but opposite in phase with respect to-the voltage between the control grid and cathode multiplied by the amplification factor of the tube, and a series resistance Rp, which represents the equivalent anode-to-cathode dynamic resistance of the tube. Thus, the voltage between the diagonal points K-gz and ye will represent the A. C. voltage between 94 and the cathode. The bridge arm between 91 and g4 will include inter-electrode capacitance C14, while the arm between 93 and 94 will include the inter-electrode capacitance C34. The diagonal between 91 and $73 will include the tube capacitance C13, which is of little importance.

It is characteristic of a bridge such as that shown, that if the voltage ratios between two diagonally opposite points, such as points 91 and g3, and another point, such as Kg2 are proportional respectively to the two reactances between the said diagonally opposite points and the fourth point (94) of the bridge, then the bridge is so balanced that any voltage between the fourth point g4 and the said diagonally opposite points K g2 can have no effect whatsoever upon the voltage between the other two diagonally opposite points (91 and 93). This condition is obtained, following the practice of the invention, by coupling L2 and L3 so that any voltage across L2 such as Eg, will cause a corresponding voltage between K-gz and ya, the latter voltage being designated Eg3- Thus, the impedances in the two arms of the bridge K-gz to g1 and K-gz to ga may be replaced by the voltage dividing network shown in broken-line representation designated VD, the function of which is to arrange the voltage between K-gz and ya, that is Eg3, so that it is at all times proportional to and opposite in phase with the voltage between Kg2 and g1, that iS Egl- Considering for the moment the other two arms of the bridge, the capacitance C14 will be very small due to the fact that the control grid 91 is almost completely shielded from 74 by means of vacuum tube of the pentode type which might be used, this capacitance might amount to a few thousandths of a micro-micro-farad. On the other hand, the capacitance 034 will be relatively large, as compared with C14, due to the fact that gs and g; are closely adjacent one another and to the further fact that in general tube practice the leads to these elements are brought out of the same end of the tube envelope, whereas the lead to g1 is taken out at the opposite end. For example, in a commercial tube such as previously mentioned, the capacitance C34 may amount to several micro-micro-farads. Thus the impedance between 91 and g4 may be several hundred times as large as that between 93 and g4. For

example, in the particular case mentioned the ratio would be around 500 or 1000 to 1. Thus, it will be seen it is only necessary to provide a very small voltage Eg3 between Kg2 and g3 as compared with the Voltage Egl between Kgz and g1 to obtain the desired balanced condition of the bridge. In the above example, Eg3 would be between and of 1% of Egl. This voltage Eg3 may easily be obtained by providing the tertiary winding L3 coupled to the secondary winding L2 by a mutual inductance M23. As will be apparent, the necessary condition for balance is that the ratio of L2 to M23 should be equal to the ratio of C34 130 C14, that is It follows that the ratio of Egl to Eg3 equals the ratio of C34 to C14, that is While from a theoretical point of view it is desirable that the coefficient of coupling between L2 and L3 be unity in order to obtain perfect balance for all frequencies, for practical purposes this condition is not necessary, and while=the coefiicient of coupling between the two should preferably be as large as possible, satisfactory stabilization may be obtained even for input and output circuits having a very high LC ratio when this coeflicient of coupling is as small as 5 or provided the above relation holds true.

As will be apparent, in is highly desirable that thetube capacitance C34 should be considerably larger than C14 and for this reason it is prefer able that the tube V should include a screening element g2 which serves to screen 9'1 from 74 and thus to minimize the value C14. The use of such an element is also desirable in that it markedly increases the gain or amplification factor of the tube. Such increase in amplification, however, in the absence of the present invention, would limit the impedance of the load circuit that could be employed, which, in turn, would tend to limit the useful stage gain. By using the circuit of the present invention, these objections are overcome and it is possible to obtain optimum efiicient operation of the stage. In the practice of the invention, the presence of 92 is'further defsirable in that by minimizing C14, and thus increasing the ratio of C34 to C14, it minimizes the magnitude of the voltage to be applied to 93 and thus minimizes the effect of ya in controlling the space current to the plate. It will be seen that since g1, g2 and g3 are all interposed between the cathode and 04, each will have an effect upon the thermionic or space current in the tube, which current should be distinguished from the current flow due to the inter-electrode capacitance of the tube. As far as the space current per se is concerned, !71 Will have the greatest control over it, 92 a lesser control, and 93 an even smaller control. Since the voltage applied to Us is considerably less than that appiled to 91 in the preferred form of the invention and since as has a smaller effect upon the space current than 71, for practical purposes, the effect of ye may be neglected. It will be noted, however, that the voltage of as is opposite in phase to that of 91 and would tend to reduce the gain of the stage, were it not for the presence of 92 which renders the actual effect of as on the space current negligibly small. In the above example the control exerted by 93 might amount to or 3 of 1% of that due to m. It is preferable that $73 be so biased as to draw no space current. This may be accomplished by conductively connecting ya to ground, as illustrated. The balance obtained in the bridge of Fig. 2 is predicated upon the hypothesis that none of the arms of the bridge conduct space current, and it will be apparent that if ya, for example, were so biased as to draw space current and thus have its voltage dependent upon Egl and additionally upon the nature of its output circuit, then the conditions for balance would no longer be the same as those given above, but would, in general, be partially dependent upon the nature of this output circuit. For practical purposes, this would require that the coefficient of coupling between L2 and L3 be considerably higher than that otherwise necessary for satisfactory balance. In other words, while 573 may have some small positive or negative potential with respect to the cathode, it should be sufiiciently negative with respect to 9'2 and 94 so that it will not conduct any appreciable space current. Furthermore, under these conditions the element 93 will serve to prevent the flow of secondary electrons between 92 and 94.

In Fig. 3, there are shown several operating curves for the amplifier of Fig. 1, illustrating the transmission characteristics which may be obtained with different degrees of stabilization. In this figure, the abscissa represents the frequency of the wave signal applied to the input, while the ordinate represents the magnitude of the signal which must be applied to the input circuit in order to obtain an output signal of standard amplitude. The curve I, which it will be noted is symmetrical about the mean frequency F0, is that obtained for proper stabilization, that is the stabilization which obtains when the above equation is satisfied. Curve 3 is that obtained in the case where the amplifier is but partially stabilized and where sufficiently high impedance input and output circuits are used so that the amplifier is nearly self-oscillatory. Curve 2 is the type of curve which will be obtained where the amplifier is more'stabilized than in the latter case but less stabilized than in the former case. It will be noted that while the amplifier transmission characteristic in this instance is better than curve 3, it is unsymmetrical. A transmission characteristic of the type indicated by curve I represents the preferred form. The stage gain has been adjusted to give equal maximum gain in each case. As is known in the art, in general the gain of an amplifier is proportional, first, to the amplification factor of the tube and, second, to the impedance of the output circuit. It is therefore desirable that a tube having a high amplification factor be used and that the tube be used with high impedance circuits. Where the input and output circuits are in the form of parallel resonant circuits, such as those shown in Fig. 1,

by which a frequency selective amplifier may be obtained, there are two limiting factors in obtaining maximum impedance for the circuit.

The first is due to the effect of the tube capacitance or input susceptance. In other words, as the impedance of the tuned circuit is increased, a larger inductance and a smaller capacitance must be used. However, the minimum usable capacitance must be sufficiently large so that variations in the input and output capacity of the tube will not cause appreciable variations in the tuning of the circuit. Another limiting factor is the conductance of the tuned circuit, that is, the resistance due to the wires which form the inductance of the circuit. By minimizing this series resistance and thus the circuit inductance, the impedance of the circuit may be increased, but variations of the conductance or resistive component of the vacuum tube input admittance will have a marked effect upon the transmission characteristics of the stage, unless this equivalent tube conductance is negligible as compared with the actual conductance of the circuit itself. By means of the present invention, the dynamic admittance of the tube is minimized and higher impedance input circuits may be used, first, because the minimum external capacitance which may be used with the input circuit may be reduced, and second, because the effective conductance of the tuned circuit may be reduced. Likewise, the impedance of the output circuit may be increased. As will be apparent, the reactive component of the impedance of the output circuit will be proportional to the impedance of that circuit and therefore, it has been necessary heretofore to limit the impedance of the output circuit, in order to minimizethe effect of this circuit upon the dynamic admittance of the tube. However, since the dynamic admittance of the tube is substantially reduced to zero by means of the present invention, considerably higher output impedance circuits may be used and the over-all gain of the amplifier may be increased, and at the same time better transmission characteristics may be obtained.

In this connection, it is interesting to note that the use of such high impedance input and output circuits characterized by having a high inductance and a small capacitance, has brought to light a new characteristic of amplifier stages not heretofore known. This phenomenon is observed when the gain of the stage is controlled by variation of the bias on 91 by means of an automatic volume control (AVC) voltage. It has been found that the static susceptance of the tube will depend in part upon the bias voltage and that the static capacitance of the tube will increase to some extent as the bias voltage is increased. This effect is apparently due to the change in the location of the space charge around the cathode as the grid bias is changed, and it has been found that the use of the high impedance circuits made possible by the present invention enables variation of the tuning of the input circuit over a small range by varying the AVG voltage to thus vary the static admittance of the tube. The net result of this effect is to cause the transmission characteristic of the tube to broaden as the AVG voltage is increased, and thus the gain of the tube is reduced for automatic volume control purposes, while the selectivity of the stage increases as the gain of the stage is increased. As will be apparent, this is a desirable characteristic since, in general, it is desirable that a radio receiver be more selective on weak signals when maximum gain is requiredthan it is on strong signals when a lesser gain is necessary. It is of course not necessary to apply an AVC voltage to 91 when this phenomenon is not desired.

Whilethe circuit of Fig.1 shows the use of the invention as applied to an amplifier stage having four fixed tuned circuits and. adapted for use in an intermediate frequency amplifier, itwill be understood that the invention is not thus lim ited and that the important elements of the invention comprise the inductance L2, the inductance L3 which is coupled to- L2, the electrode and wiring capacities C14 and C34, and the elements of the tube structure which cause the capacity C34 to be considerably larger than the capacity ,C14. ,It will be apparent further that the invention is not limited to the illustrated method of obtaining the voltage E33, since this voltage may be obtained in any desired manner so long as it is properly related in amplitude and phase to Egl. Itis believed, however, that the illustrated method of obtaining this voltage is the simplest and most economical method since only a few turns of wire are required.

In one practical embodiment of the invention as shown in Fig. l, which constituted part of an intermediate frequency amplifier adapted to transmit audio-modulated signals having a carrier frequency of 4'70 k. c., the impedance of L1 by itself was about 2500 ohms, while that of each of L2, L4 and L5 individually was about 5000 ohms (at 470 k. 0.). V1 was a commercial pentode amplifier of the type known as 6K7G, operating with a plate voltage of about 300 volts and a screen voltage of about 100 volts. The primaries and secondaries of T1 and T2 were each tuned to 470 k. c. by padding condensers and the coils were slightly less than critically coupled. The Q of the primaries and secondaries of each transformer was in the neighborhood of 175. The inductance L2 was in the form of a bank- Wound coil made up of 6 banks totaling 316 turns of Number 7-41 Litzendraht single silk enameled wire, while the tertiary coil L3 comprised 2% turns of Number 30 double silk covered wire wound in solenoid form spaced 4; of an inch from the secondary, both coils being wound on a form. While the proper number of turns for exactly stabilizing the amplifier may be computed-from the relation given above, for practical purposes the proper number of turns may be.

more easily determined by first padding the transformer T2 tothe desired frequency, then replacing the transformer T2 by a small resistive load and padding the transformer T1 and finally re-inserting the transformer T2 and determining if the transformer T1 is still properly padded.

If the stage is properly balanced, it will be found that the transformer is correctly padded. If the additional turns are needed for L3 to balance the stage,it will be necessary to reduce the capacitance of T1 to properly pad the circuit. On the ode, a control grid, 2. screen grid and a third grid, said grids being so positioned with respect to said anode as to provide a certain capacitance between said control grid and said anode and a greater capacitance between said third grid 1 and said anode, an output circuit including an impedance and a source of electrical energy connected to said anode and said cathode, means biasing said screen grid to a positive potential with respect to said cathode, means for supplying an input signal between said cathode and said control grid, and means for applying a signal of opposite phase with respect to said input signal between said cathode and said third grid, and for biasing said third grid so as to prevent it from drawing any substantial space current, the ratio of the amplitude of said first signal to that of said second signal beingsubstantially equal to the ratio of said second capacitance to said first capacitance.

2. An amplifier for wave signals, comprising a space discharge device having an anode, a cathode, a control grid, a screen grid and a third grid,

.said control grid being adjacent said cathode,

said screen grid and said third grid being positionedbetween said control grid and said anode,

thereby providing a capacitance between said control grid and said anode, and a much greater capacitance between said third grid and said anode, an output circuit including an impedance and a source of electrical energy connected to said anode and said cathode, means biasing said screen grid to a positive potential with respect to said cathode, means for supplying an input signal between said cathode and said control grid, and means for applying a signal of opposite phase with respect to said input signal between said cathode and said third grid, and for biasing said third grid so as to prevent it from drawing substantial space current, the ratio of the amplitude of said first signal to that of said second signal being substantially equal to the ratio of said second capacitance to said first capacitance.

3. An amplifier for wave signals, comprising a space discharge device having an anode, a cathode, a control grid, a screen grid and a third grid, said control grid being adjacent said cathode, said screen grid and said third grid being positioned between said control grid and said anode, thereby providing a capacitance between said control grid and said anode and a greater capacitance between said third grid and said anode, an output circuit including an impedance resonant at one frequency and a source of electrical energy connected to said anode and said cathode, means biasing said screen grid to a positive potential with respect to said cathode, means including a resonant circuit tuned to said one frequency and having an inductive element adapted to be energized by a source of input signals for supplying said signals between said cathode and said control grid, and means including a second inductance coupled to said first inductance for applying a signal of opposite phase with respect to said input signal between said cathode and said third grid, and for biasing said third grid so as to prevent it from drawing any substantial spacecurrent, the ratio of said first inductance to themutual inductance between said first and said second inductances being substantially equal to the ratio of said second capacitance to said first capacitance.

4. An amplifier for wave signals, comprising a space discharge device having an anode, a cathode, a. control grid, a screen grid and a third grid, said grids being so positioned with respect to said anode as to provide a certain capacitance between said control grid and saidanode and a greater capacitance between said third grid and said anode, an output circuit including an impedance and a source of electrical energy connected to said anode and said cathode, means biasing said screen grid to a positive potential with respect to said cathode, means including an inductance adapted to be energized by a source.

of input signals for supplying said signals between said cathode and said control grid, a second inductance coupled to said first inductance, and .a connection between said third grid and said cathode through said second inductance, for applying a signal of opposite phase with respect to said input signal between said cathode and said third grid, and for providing a low impedance path for unidirectional current, the ratio of the amplitude of said first signal to that of said second signal being of the same order of magnitude as the ratio of said second capacitance to said first capacitance.

5. An amplifier for Wave signals, comprising a space discharge device having an anode, a cathode, a control grid, a screen grid and a third grid, said control grid being adjacent said cathode, said screen grid and said third grid being positioned between said control grid and said anode, so as to provide a capacitance between said controlgrid and said anode and a greater capacitance between said third grid and said anode, an output circuit including a resonant impedance and a source of electrical energy connected to said anode and said cathode, means biasing said screen grid to a positive potential with respect to said cathode, means including an inductance adapted to be energized by a source of input signals for supplying said signals between said cathode and said control grid, a second inductance coupled to said first inductance, and a connection between said third grid and said cathode through said second inductance, for applying a signal of opposite phase with respect to said input signal between said cathode and said third grid, and for providing a low impedance path for unidirectional current, the ratio of the amplitude of said first signal to that of said second signal being of the same order of magnitude as the ratio of said second capacitance to said first capacitance.

ERNEST C. FREELAND. 

